Hashcash should use stronger hash

[paulpach]

Describe the bug
FNV is not suitable for Hashcash.

Ruffles/Ruffles/Hashing/HashCash.cs

Line 23 in 3f0c582

return (difficulty == 0 || ((HashProvider.GetStableHash64(challenge + additionsRequired) << ((sizeof(ulong) * 8) - difficulty)) >> ((sizeof(ulong) * 8) - difficulty)) == 0);

Any math undergrad can break this in his spare time.
You need a stronger cryptographic algorithm for hashcash. The state of the art appears to be sha256, or at least sha1:
https://en.bitcoin.it/wiki/Hashcash#Hash_function_choices